With electric vehicles, fuel cell vehicles and hybrid vehicles, an electric motor as a motor is used as the power source or the auxiliary power source. As the power source of this electric motor, a secondary battery in which numerous single cells having a storage battery are connected in series is adopted. In particular, if numerous lithium ion batteries in which the battery voltage of the single cells is high are connected in series, it is possible to achieve high voltage and high output. Thus, in recent years, the development of a battery system adopting a battery pack in which signal cells configured from a lithium ion battery are connected in series is becoming popular.
As conventional technology, for example, a device for detecting the voltage of a battery pack for an electric vehicle having a main battery configured from numerous battery modules for storing running power at a high voltage of 300 V, and an auxiliary battery to be used for auxiliary drive has been proposed. This voltage detection device has a voltage detection circuit for individually detecting the voltage from the respective battery modules, and a signal processing circuit for processing signals that indicate the respective module voltages detected with the voltage detection circuit, and the voltage detection circuit uses the power supplied from the auxiliary battery via a DC-DC converter as the drive power (refer to Patent Document 1). Thus, it is possible to realize high accuracy upon detecting the open-circuit module voltage, and avoid the unwanted decrease in the life of the high voltage battery pack.
As another conventional technology, proposed is a power storage device configured by connecting single batteries of a plurality of secondary batteries in series to form a power storage module and arranging a plurality of such power storage modules for use in an electric vehicle, wherein the respective power storage modules comprise a controller for controlling the single batteries provided to the respective power storage modules (refer to Patent Document 2).
The controller of the power storage device seeks the logical sum of the detected voltage of the single batteries in the power storage module and the voltage from the high voltage potential-side controller, and outputs the result to the low voltage potential-side controller. The respective controllers comprise a voltage detection circuit for detecting the voltage of the signal battery in the power storage module to be controlled, and a superordinate controller for controlling the foregoing controller based on the voltage detected with the voltage detection circuit is also provided.
In addition, a battery protection IC for monitoring the module configured by connecting a plurality of single batteries in series is also being proposed (refer to Patent Document 3), and this is provided with an overvoltage detection circuit for determining an overcharge condition and outputting an overvoltage signal if a terminal voltage of any of the batteries in the module becomes a predetermined value or higher when the battery is being charged, and an over discharge detection circuit for determining an over discharge condition and outputting an over discharge signal if a terminal voltage of any of the batteries in the module becomes a predetermined value or lower when the battery is being discharged. Moreover, this battery protection IC also has a first switch capable of issuing a warning by being turned on when an overvoltage signal has been detected, and a second switch capable of issuing a warning by being turned on when an over discharge signal has been detected.    Patent Document 1: Japanese Published unexamined Application No. H11-160367    Patent Document 2: Japanese unexamined Patent Application No. 2003-70179    Patent Document 3: Japanese unexamined Patent Application No. 2005-117780
Meanwhile, as a result of using the foregoing conventional technologies, a battery system configured from a battery pack in which numerous lithium ion batteries are connected in series is provided with a voltage detection circuit for monitoring the voltage of the respective single cells, and this voltage detection circuit controls the generation of an overcharge or over discharge by monitoring the battery voltage.
Here, with a battery system configured from numerous lithium ion batteries, numerous wirings are required for connecting the single cells in the module and for connecting the modules. In addition, wiring for a large current up to the output terminal of the battery system is also required, and wiring from the single cells to the voltage detection circuit is also required.
Thus, when mounting a battery system with numerous components built therein and connected with wirings on a mobile object such as an automobile, since the battery system will be used for a long period of time in traveling conditions with continued vibration, there is a possibility that the components may become misaligned or a short circuit may occur due to the disconnection of wirings or the friction of components. Consequently, an abnormal voltage or current may occur in a circuit inside the voltage detection device configuring the battery system, and as a result, the circuit components and the like may ignite or generate smoke, and considerably hinder the driving operation of the automobile.
In particular, if the connection between the cells in the module having a voltage detection circuit for detecting the voltage of each cell is disconnected, an abnormal voltage exceeding the withstand voltage of the voltage detection circuit may be applied to the voltage detection circuit, to cause a disconnection in the detection circuit or the occurrence of a short circuit, and as a result, the circuit components or the printed pattern in the circuit substrate may generate heat and smoke and, in certain cases, may even ignite.
For example, as shown in FIG. 10(a), in a battery system configured by arranging a plurality of modules in which 10 cells are connected in series and having a battery voltage measurement circuit for measuring the voltage of the respective cells in the respective modules, if the cells in the module are disconnected during a discharge, the circuit voltage that is applied to the battery voltage measurement circuit can be expressed with the following expression.Voltage of measurement circuit=(Module voltage)−(Pack voltage)  [Expression 1]Provided that:Pack voltage: voltage of all modules in battery system
Here, when taking an example of a battery pack using a lithium battery in which the voltage of each cell is 3.6 V and the pack voltage is generated by 10 modules, since 10 cells are connected in series in each module, the voltage of each module will be 36 V, and since a total of 100 cells of the 10 modules are connected in series, the pack voltage will be 360 V. Thus, as shown in FIG. 10(b), a voltage of −324 V is applied to the battery voltage measurement circuit. Consequently, the discrete element or IC configuring the measurement circuit will be destroyed to cause a short circuit current to flow, as a result, the circuit components, IC, circuit pattern and the like may generate heat, generate smoke or ignite.
Meanwhile, as shown in FIG. 11(a), if the cells in the module are disconnected while charging the battery pack via a charger, the circuit voltage that is applied to the battery voltage measurement circuit can be expressed with the following expression.Voltage of measurement circuit=(Maximum voltage of discharger)−{(Pack voltage)−(Module voltage)}  [Expression 2]
Here, as with FIG. 10, when taking an example of a battery pack using a lithium battery in which the voltage of each cell is 3.6 V and the pack voltage is generated by 10 modules, and the maximum voltage during the load open-circuit of the charger is 550 V, since a total of 100 cells of the 10 modules are connected in series, the pack voltage will be 360 V, and the voltage of each module configured from 10 cells will be 36 V. Thus, as shown in FIG. 11(b), a voltage of 226 V will be applied to the battery voltage measurement circuit. Consequently, as with the case during a discharge, the discrete element or IC configuring the measurement circuit will be destroyed to cause a short circuit current to flow, as a result, the circuit components, IC, circuit pattern and the like may generate heat, generate smoke or ignite.
The present invention was devised in order to overcome the foregoing problems. Thus, an object of this invention is to provide a battery system capable of preventing the voltage that is applied to the voltage detection circuit from exceeding the withstand voltage of the voltage detection circuit when the connection between the cells in the battery module is disconnected, and preventing an overcurrent from flowing in the cell voltage detection circuit and the module.